This invention relates generally to non-destructive inspection of structures and, more particularly, to methods and apparatus for non-destructive inspection of monocoque and/or semi-monocoque structures which utilize excitation of nodal patterns in the skin of the structure and recording such patterns.
A semi-monocoque structure, such as an aircraft fuselage, is basically a skeleton over which a skin is secured, and may be characterized as a frame-lined shell. The skin, usually formed of metal which may vary in thickness or stiffness according to design requirements, is fixed to the skeletal framework so as to form a series of panels. It is from the skeletal framework surrounding each panel that the skin derives its strength. If the skin develops cracks, or is weakened or becomes detached from the skeletal framework, then a rupture can result causing impairment of the safety of the entire structure, particularly if the system is operated at pressure. Similarly, if the underlying structure cracks, or become seriously corroded, or the fastening system fails, structural failure can result. Since the semi-monocoque structure of an aircraft fuselage is usually pressurized, the fastening of the skin to the skeleton is critical, as is the integrity of the skeleton itself. Therefore, it is important that inspection techniques ascertain not only the condition of the skin and the fixers, such as rivets used to attach the skin to the framework, but also that of the underlying or sub-surface elemental skeleton.
However, it is very difficult to inspect the substructure of the skin of an aircraft fuselage for corrosion, broken fastenings, cracks or loose rivets, a situation referred to in the aircraft industry as "multi-element damage". Eddy current and acoustic systems heretofore used for the purpose achieve less than acceptable results. Visual inspection is a commonly used method, but substructural elements of a monocoque structure can be inspected with assurance only by using fiber-borescopes, or by stripping out interior instrumentation, linings or, in cases where a fault is suspected, making an X-ray examination, all of which procedures are time-consuming, expensive, and far from foolproof in their ability to detect faults.
Holographic interferometry has also been used to detect material flaws or nonhomogeneities through their effect on surface deformation when the test object responds to some mechanical or thermal load. By this technique complex laser light waves can be recorded and later reconstructed with such fidelity that they can be used to form interference fringe patterns. Using a pulsed laser, operated in a manner to produce two sequentially timed pulses and two holographic records--one of the light of the first pulse scattered by an object in some initial or known state and the other of the light from the second pulse scattered by the same object while it is subjected to vibration, for example--an image of the object with superimposed interference fringes is formed. These fringes are contour curves of constant surface displacement and provide a display of the deformation of the test object in response to the applied load. As this deformation may be affected by the presence of surface cracks or sub-surface defects, by comparing the fringe patterns for a flawed test object with those for a defect-free object it is possible to detect the presence of a subsurface defect, and possibly to estimate its size and depth.
It is also known that debonds and delaminations can be detected by inducing resonance of the locally flexible region created by the flaw. By using real-time holographic interferometry the surface response can be observed as the transducer which drives the vibration is tuned through a range of frequencies. This is accomplished by observing the vibrating object through a hologram which previously was recorded while the object was stationary. Vibrational techniques have been used to detect flaws in a variety of laminates and honeycomb materials as well as structures and components including airframe panels, turbine blades, helicopter rotors, cathode ray tube face plates and brake disks.
Interpretation of the fringe map of contour patterns is usually qualitative, consisting of searching for abrupt changes of curvature near cracks or for closely spaced "bull's eye" fringes near debonds, or fringes in normally fringe-free areas of low fringe count such as stringer or frame lines; this usually requires some operator judgment.
While the basic principles and techniques of holographic nondestructive evaluation are well-developed, to applicant's knowledge holographic NDE has not been satisfactorily utilized for the reliable detection of flaws in the substructure of monocoque or semi-monocoque structures. Accordingly, a need exists for apparatus and methods for nondestructively inspecting for multi-element damage in monocoque or semi-monocoque structures, such as aircraft fuselages.
The present invention utilizes a vibrational nondestructive technique for locating damage and flaws in such structures. Applicant's studies have shown that a semi-monocoque structure, when excited by a low energy vibrator attached to the skin, may freely vibrate at a number of resonant frequencies which have very narrow bandwidths, on the order of two or three cycles, compared to the separation of the frequencies. The panels circumscribed by the skeletal substructure resonate in a uniquely characteristic pattern of nodes and anti-nodes, and if the frequency of excitation, or the structural support of a panel is changed, then the nodal pattern changes shape, frequently becoming asymmetric, or in the extreme, changing frequency. The nodal patterns differ markedly in dependence on whether the excitation is applied at a node or is applied at an anti-node, for a particular frequency of excitation. In order for the panel to take up its own natural resonance and nodal pattern shape in a steady and constant manner, excitation must be applied at an anti-node.
A recording of changes in the nodal pattern can nondestructively reveal hidden flaws in the substructure and/or in the skin of the structure. However, applicant has recognized from results of his described studies that a recording of nodal pattern changes can reveal flaws with acceptable accuracy only if the recording is made at the instant certain conditions are present in the nodal pattern; these conditions are maximum displacement of the anti-nodes and satisfaction of certain conditions of phase of the anti-nodes and correct excitation.
Accordingly, a general object of the invention is to improve the accuracy of holographic nondestructive inspection methods and apparatus using vibrational techniques for the detection of flaws in monocoque and semi-monocoque structures.
A more specific object of the invention is to provide a method and apparatus for recording two time-displaced overlaying holograms of the vibrating skin of a monocoque structure at such times that the interference pattern generated by reconstruction of the holograms will optimally reveal flaws in the structure.
Another object is to provide a method and apparatus for correlating phase and amplitude of anti-nodal information in a vibrating structure to diagnose the structural condition of a monocoque structure.